1. Field of the Invention
The present invention relates to detecting a size and a position of a paper. The present invention is more particularly related to a document detecting apparatus of an image forming apparatus for detecting a size and a position of a document based on directing incident light to the document and detecting light reflected therefrom.
2. Discussion of the Background
In an image forming apparatus such as a copying machine, it is helpful if a scanning range of an optical system is set and a paper sheet cassette is selected automatically. For this reason, a document detecting apparatus is installed in an image forming apparatus for detecting a size and position of a document by directing incident light to the document and detecting reflected light from the document.
Conventionally, this type of document detecting apparatus has problems resulting from a complicated structure and that an operator may mistakenly operate the apparatus by not distinguishing preliminary light scanning and exposing scanning. Further, a pulsating disturbance light, such as an incandescent lamp is not completely prevented from interfering with detection of incident light reflected from the document.
Whereas, Japanese Laid Open Patent No. 1990-308236 discloses a document size and position detecting apparatus, in which the document size and position is detected accurately and rapidly by a control device, on the basis of information obtained from twice light scanning by a light scanning device which is operated by a closing operation of a document pressing board which fixes the document on a contact glass. The above described patent is further of a simple structure in which wiring of an exposure scanning unit is prevented.
Moreover, Japanese Laid Open Patent No. 1992-67136 discloses a document detecting apparatus in which an on/off controlled impulse light is directed to a document from a light emitting block and an output signal outputted from a light detecting block is filtered in a high pass active filter, and thereby direct light disturbances of a direct current such as a sun light and pulsating light disturbances such as from an incandescent lamp etc. are prevented from being included therein.
Moreover, Japanese Laid open Patent No. 1989-186923 discloses a document size detecting apparatus in which existence of a document is determined by comparing a light detecting signal from a light sensor which is obtained by scanning a document with reference to a predetermined value, and the size of a document is detected based on a measurement timed in accordance with when the light detecting signal changes from a logic value "1" to "0".
Further, a document detecting apparatus is also proposed in which a light detecting signal distribution is acquired by scanning when a light emitting source is on and a second light detecting signal distribution is acquired by scanning when the light emitting source is off. A difference between the first and second signal distributions is calculated, and thereby a document detecting signal is determined with disturbance light influences removed.
In Japanese Laid Open Patent No. 1990-308236, it is necessary that document detecting is executed during a short time while the document pressing board is closed. In order to obtain enough resolution for high accuracy document detecting, high performance parts and a complicated design are required, and thus an apparatus becomes complicated and manufacturing costs are increased.
In Japanese Laid Open Patent No. 1992-67136, the disturbance light is removed effectively when in steady state, however, during scanning, rapid changes in incident light due to disturbances pass the filter and are detected, and further, with respect to some reflecting light, detection becomes unstable by a transient response property of the filter.
Moreover, in some cases, for example, even when a light emitting source is driven at a duty ratio of 50%, a duty ratio of a received binary signal may be shifted largely by properties of an optical system and a circuit system receiving the binary signal. In this case, according to a phase relation between a clock and a received output signal, light reflected from the document is not properly received preventing document detection. Further, on the basis of an on/off driving of the light emitting source, an error range generates at an edge portion of the document that may prevent detection of the edge portion of the document.
Moreover, in the apparatus in which the light detecting signal distribution is acquired by scanning when the light emitting source is on and light detecting signal distribution acquired by scanning when the light emitting source is off, an A/D converter and a large capacity memory are necessary, and thus an apparatus becomes complicated and of large size. Further, signal processing becomes complicated and processing time is extended.
And finally, a small size and low torque motor is often used for driving an optical system of a document detecting apparatus, because of a small size and a simple structure of such apparatus. However, if rotation of the motor becomes abnormal by some cause, proper scanning by the optical system cannot be executed, preventing proper document detection.